The present invention relates to irradiated polyolefin film and more particularly to a process of irradiating polyolefin film to create substantially uniform dosage throughout the width of the film.
Polyolefins and particularly polyethylene have been widely used for various functions. A particular use of such material has been in the form of a film or sheet of material used in packaging. Such material is frequently oriented so as to be heat shrinkable around articles packaged therein.
In order to enhance the properties of polyolefins and particularly polyethylene, such materials are crosslinked by irradiation. An example of such an irradiation process is disclosed in U.S. Pat. No. 3,144,399 to Rainer et al. Such an irradiation process comprises subjecting the polymer to an irradiation dosage of about 1 to about 20 megarads.
In the process of producing oriented polyethylene film a tape of extruded material is subjected to such irradiation and is then heated and oriented by known techniques. The technique for so doing is described in U.S. Pat. No. 3,741,253 as it relates to the irradiation and orientation of a copolymer of ethylene and vinyl acetate.
A problem, however, of producing irradiated film is that for some reason the edge area of a film tends to be significantly less crosslinked than the central area of the film, i.e., for some reason the effective irradiation dosage at the film edge is less than the effective dosage within the central portion of the film. This obviously results in a film having non-uniform properties across the width thereof. A technique is disclosed in U.S. Pat. No. 2,993,120 for magnetically concentrating electrons around a thin film during the irradiation process. This process, however, while increasing the efficiency of electron irradiation, does not solve the problem of nonuniform dosages across the width of the film.